Domain ontologies are formal descriptions of the classes of concepts and the relationships among those concepts that describe an application area. The Protege software-engineering methodology provides a clear division between domain ontologies and domain-independent problem- solvers that, when mapped to domain ontologies, can solve application tasks. The Protege approach allows domain ontologies to inform the total software-engineering process, and for ontologies to be shared among a variety of problem-solving components. We illustrate the approach by describing the development of EON, a set of middleware components that automate various aspects of protocol-directed therapy. Our work illustrates the organizing effect that domain ontologies can have on the software-development process. Ontologies, like all formal representations, have limitations in their ability to capture the semantics of application areas. Nevertheless, the capability of ontologies to encode clinical distinctions not usually captured by controlled medical terminologies provides significant advantages for developers and maintainers of clinical software applications.
